The expressions “energy scavenging” and “energy harvesting” refer to technologies, known in the art, for converting ambient, stray energy, available from external sources, into electricity. For example, a piezoelectric crystal can be used to convert mechanical energy from vibrations into electric energy. As another example, a temperature difference between two different electrical conductors can be used to generate a voltage across, or current through, the junction of the conductors. As still another example, consider a magnet that is made to move past a coil as a result of the vibratory or cyclic motion of a mass to which the magnet is attached. The moving magnet generates a change of magnetic flux in the coil, and induces an electromotive force on the coil. Accordingly, a plurality of technologies is available for extracting energy from the environment. A component, which extracts stray energy from its environment and makes the extracted energy available for consumption, is referred to as an energy scavenger.
For completeness, the expression “energy scavenger” as used throughout this text and the claims, is also meant to cover the concept of an energy harvester, for anyone who would like to make this distinction. An energy scavenger takes energy from the side effects of a process in a dynamic system. Consider, for example, deriving energy from the heat that is produced by the unavoidable friction in a running rolling-elements bearing or a plain bearing). In practice, taking energy from the side effects hardly affects the process itself, if at all. An energy harvester takes energy from the process itself and thereby influences the system. In principle, the energy drained from the process needs to be replaced by the power source driving the process. Consider, for example, the dynamo installed on a bicycle to power the bicycle's headlight and rear lamp. The dynamo is driven by the bicycle's rotating front wheel or rear wheel that in turn is driven by the rider him/herself via the pedals.
The extracted energy is stored in, e.g., a rechargeable battery or a supercapacitor. A rechargeable battery is also referred to as a secondary cell. A supercapacitor is also known as an electric double-layer capacitor. A supercapacitor is an electrochemical capacitor that has a much higher energy density than a conventional capacitor, owing to the use of a layer of nano-porous material that dramatically increases the layer's surface area, allowing many more charge carriers to be stored in any given volume.
A typical application of using an energy scavenger is the powering of small autonomous electronic devices, e.g., sensors, or actuators. These devices are small and require little power. Their applications are limited by the availability of electric power. Scavenging energy from ambient vibrations, wind, heat, light, etc., enables to replenish the energy drawn from the energy storage for consumption by the electronic device, so as to keep the device functioning over a time span that is long in view of the amount of energy available from the energy storage in the absence of the energy scavenger. Such devices can be exploited in, e.g., condition monitoring applications.
For examples of energy harvesting for wireless sensors see, e.g., U.S. patent application publication No. 20050140212, or U.S. patent application publication No. 20080047363. The harvested energy is stored in a rechargeable battery.